In medical radiography a patient is exposed to X-radiation, and the pattern of X-ray attenuation by the patient is recorded in a radiographic film. When processed, a visible silver image is produced in the radiographic film that can be employed as a diagnostic aid in medical treatment.
Frequently a duplicate of the image captured in the radiographic film is required. The simplest approach for accomplishing this is to expose a directpositive radiographic film through the silver image in the original (a.k.a., taking) film. Since the silver image in the taking film is almost invariably a negative image, the duplicating film produces a second negative image.
Although there are two commonly used types of direct-positive silver halide emulsions, (1) the internal image desensitization type and (2) the surface fogged type, only the latter type is compatible with the photographic processing solutions used for radiographic taking films, and hence the latter is the emulsion type of choice for radiographic duplicating films.
Historically a fundamental difficulty in attempting to use surface fogged silver halide emulsions to construct radiographic duplicating films has been that a single emulsion cannot satisfy exposure latitude requirements. The exposure latitude requirement of the duplicating film arises in the following manner. To produce a duplicate of the original or taking film image, it is necessary that the duplicating film exhibit an average contrast of approximately -1.0, duplicating film average contrasts having absolute values (i.e., ignoring the sign) of less than .vertline.1.0.vertline. will decrease the average contrast of the duplicated image and duplicating film average contrasts of absolute values greater than .vertline.1.0.vertline. (again, ignoring the sign) will increase the average contrast of the duplicated image. Average contrast is the quotient of the following relationship: EQU .gamma..sub.av. =.DELTA.D.div..DELTA.log E
where
.gamma..sub.av. =average contrast; PA1 .DELTA.D=the change in optical density; and PA1 .DELTA.log E=the change in exposure, E being measured in lux-seconds. PA1 D is a chromophoric light-absorbing compound, which may or may not comprise an aromatic ring if y is not zero and which comprises an aromatic ring if y is zero; PA1 A is an aromatic ring bonded directly or indirectly to D; PA1 X is a substituent, either on A or an aromatic ring portion of D, with an ionizable proton; PA1 y is 0 to 4; and PA1 n is 1 to 7. PA1 R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are the same or different substituted or unsubstituted alkyl or aryl groups, one or more of R.sub.1, R.sub.2, R.sub.3 and R.sub.4 contains carboxy substituent --C(O)O.sup.31 Z.sup.+, wherein Z.sup.+ is a statistical mixture of hydrogen (H.sup.+) and alkali or tetraalkylammonium cations (M.sup.+) such that Z.sup.+ =xH.sup.+ +(1-x)M.sup.+, where x is a decimal ranging from about 0.33 to about 0.95.
Notice that when density decreases with increasing exposure .DELTA.D is a negative number and .gamma..sub.av. is also a negative number. The contrasts of negative-working films are positive values while the contrasts of direct-positive films are negative values. In both negative-working and positive-working films the higher the absolute value of contrast (ignoring the sign) the higher the contrast. Since radiographic taking films typically have image densities ranging from &gt;3.0 to 0, .DELTA.D in a duplicating film must at least approach -3.0. For average contrast, .gamma..sub.av., to be approximately -1.0, exposure latitude, .DELTA.log E, must also be approximately 3.0.
There is no single surface fogged direct-positive emulsion that exhibits an exposure latitude of 3.0 log E. As taught by Illingsworth U.S. Pat. Nos. 3,501,305, '306 and '307, the most efficient direct-positive emulsions are those produced by surface fogging silver halide grains that are regular and monodisperse. This allows a greater percentage of the total grain population to receive optimum surface fogging. Unfortunately, emulsions with monodisperse grain populations exhibit only narrow exposure latitudes.
It is common practice to blend emulsions of differing mean equivalent circular diameter (ECD) grains to increase exposure latitude. Using monodisperse emulsions having a mean coefficient of variation (COV) of less than 20% (taught in other terms by Illingsworth U.S. Pat. No. 3,501,305), it is burdensome to separately precipitate, fog and blend the many different emulsions required to achieve an exposure latitude of 3.0 log E.
Another approach to increasing the exposure latitude of direct-positive emulsions is to blend surface fogged silver halide grains that have been fogged to different degrees. The problem is that the range of surface fogging differences to achieve an exposure latitude of at least 3.0 log E results in an emulsion blend lacking satisfactory levels of sensitometric stability.
Taber et al U.S. Pat. No. 3,647,463 illustrates how the art has struggled to work within the limitations of fogged direct-positive emulsions to construct radiographic duplicating films of the required exposure latitude. Taber et al discloses the following structures:
______________________________________ Gelatin Layer ______________________________________ Light-Sensitive Silver Halide (13) Light-Insensitive 0.1-1.5 Density Layer (12) Light-Sensitive Silver Halide (11) Support FIG. 1 ______________________________________
______________________________________ Gelatin Layer ______________________________________ Light-Sensitive AgX (16) Light-Insensitive 0.1-1.5 Density Layer (15) Light-Sensitive Layer + Absorbing Dye (14) Support FIG. 2 ______________________________________
______________________________________ Gelatin Overlayer ______________________________________ Light-Sensitive Layer (18) Support Light-Sensitive Layer + Absorbing Dye (17) Gelatin Overlayer FIG. 3 ______________________________________
In Example 4 of Taber et al an exposure latitude of 2.5 at an average contrast of -1.14 and a maximum density of 3.31 is achieved using the FIG. 2 construction, in which a total of three different emulsion layers (14, 15 and 16) are coated, with emulsion layer 14 containing a blend of three different emulsions and two absorbing dyes, layer 15 containing a single unsensitized emulsion, and layer 16 containing a blend of two different emulsions. The remaining arrangements reported in the Examples, although almost equally complex, all fail to satisfy acceptable performance requirements for radiographic duplicating film.
Inoue et al U.S. Pat. No. 5,298,381 discloses a photographic element containing a direct-positive emulsion layer having surface fogged grains and an overcoat layer containing a microcrystalline dye employed for the purpose of imparting room light handling capability to the element.
Microcrystalline dyes are known to reduce crossover in dual coated (e.g., Duplitized.TM.) radiographic taking films when coated between the emulsion layer units and the transparent film support, as taught by Dickerson et al U.S. Pat. Nos. 4,803,150, 4,900,652 and 4,997,750.
Further illustrations of microcrystalline dyes contained in photographic elements are provided by Factor et al U.S. Pat. No. 4,855,221, Diehl et al U.S. Pat. Nos. 4,877,721 and 4,940,654, Anderson et al U.S. Pat. No. 4,988,611, Usami et al U.S. Pat. No. 5,238,799, Texter U.S. Pat. No. 5,274,109 and Karino EPO 0 456 163.